Several capacitive display devices are known in the art. These include electroluminescent displays, plasma displays, and field emission displays. Each pixel in a capacitive display device can be modeled as a capacitor, and the interconnections can be modeled as resistances. Because the display can be modeled as a distributed resistive-capacitive network, each addressed row and column has an intrinsic low-pass filter characteristic and bandwidth. Driving one end of a row or column with a signal having a bandwidth that partially lies outside the bandwidth of the device will result in that signal becoming increasingly filtered as it travels across the display. The signal at each pixel along the given row or column will differ from the inputted drive signal and from the signal at the other pixels. This signal distortion contributes to visual image distortion. The pulse at the pixels closest to the driver are sharp; the pulse at the pixels farther from the driver are slower and filtered and produce a lower intensity image. The result is a gradient in brightness across the display for a given drive signal.
One prior art scheme for correcting drive signal distortion due to signal filtering includes applying a corrective pulse width modulation (PWM) to the drive signal. This corrective PWM may be employed in addition to PWM that is responsive to grey level data. In this prior art scheme, the extent of the corrective PWM depends upon the destination of the drive signal within the matrix of the display. The corrective PWM circuitry is responsive to indications of the row and the column being addressed by the drive signal. PWM allows simple circuitry. However, PWM requires a high frequency modulation clock. This frequency increases with increasing number of grey shades and with higher display resolution. At these higher frequencies the pulse transition times from "on" to "off" and back become a significant factor in display performance. For example, the rise time and fall time of the drive signal can become greater than the active time of a pixel. This can result in no image being displayed. Furthermore, this prior art scheme has the disadvantage of requiring additional data processing to determine the corrective PWM for each pixel location.
Accordingly, there exists a need for an improved method for driving a capacitive display that allows higher resolution, provides a greater number of grey shades, is simple to implement, and reduces data processing requirements.